Determination of the Solute Content and Volumetric Properties of Binary Ionic Liquid Mixtures Using a Global Regularity of Molar Volume Expansion

The
hygroscopic nature of many ionic liquids (ILs) is at the core
of the disagreements in the literature data reporting their physicochemical
properties. The customary experimental procedures to measure the impurities
present in pure ILs usually entail certain technical challenges and
are commonly relatively costly. In the current study, a global regularity
of the molar volume expansion (Δν/ν) is proposed
based on an examination of 1371 isotherms of 305 binary systems with
74 distinct ILs and 15 polar and nonpolar solutes, comprising a total
set of 14,241 datapoints. This regularity is presented for the first
time as a robust alternative for the representation of experimental P–ν–T–x data of binary IL+ solute systems, with a much simpler
formulation compared to different equations of state and without the
inclusion of any temperature-dependent adjusted parameters. It is
presented in two global as well as system-specific forms, and in doing
so, the proposed regularity can be useful for a number of different
purposes. First, it essentially makes it possible to readily determine
solute composition of a mixture from mass density measurements alone.
For instance, the water content, which is of great significance in
IL studies, can easily be estimated using the proposed algorithm.
By doing so, an overall AARD of 3.47% was obtained for the estimation
of the water content of 68 binary systems. Second, with this principle,
it is possible to predict the molar volume of a binary mixture, given
the solute composition. Also, third, the regularity of interest is
very handy for determining the densities of completely pure ILs, which
are not always easily measurable, especially for those ILs that have
a high affinity to impurities, including but not limited to atmospheric
humidity. This application was undertaken here, resulting in an overall
AARD of 1.02% for the ILs considered in this study. Despite providing
such various applications, a notable feature of the proposed model
is that it covers mixtures with all types of solutes, ranging from
nonpolar to highly polar, combined with all groups of ILs, and essentially
stipulates no restriction on the type of systems that it encompasses.
Overall, the total AARD values of 0.18, 0.38, and 2.80% were obtained
for estimation of the mixture molar volume, alongside the solute compositions
that were estimated by employing either molar volume or mass density
data as an input for the system-specific model, respectively. Similarly,
the abovementioned deviations were 1.02, 0.49, and 4.33%, respectively,
utilizing the proposed global model. Besides, a brief comparative
study reveals that the proposed model has a favorable performance
comparable to the Redlich–Kister expansion in the representation
of the molar excess volume of binary IL mixtures.